1. Field of the Invention
The present invention relates generally to a broadband wireless access (BWA) communication system, and in particular, to a system and method for implementing a handoff as determined by a subscriber station (SS) in a traffic state in a BWA communication system using OFDM (Orthogonal Frequency Division Multiplexing).
2. Description of the Related Art
Currently, a number of studies are being conducted on providing of services with diverse QoSs (Qualities of Service) at or above about 100 Mbps to users in a next generation mobile communication system, i.e., a 4th (4G) generation communication system. The existing 3G communication systems support about 384 Kbps in an outdoor channel environment that is relatively poor and up to 2 Mbps in an indoor channel environment that is relatively good. Wireless LAN (Local Area Network) systems and wireless MAN (Metropolitan Area Network) systems generally support 20 to 50 Mbps. In this context, a new communication system is being developed by guaranteeing mobility and QoS to the wireless LAN and MAN systems supporting relatively high data rates, thereby supporting high-speed services intended in the 4G communication system.
Despite its feasibility for high-speed communication service due to a wide coverage area and a high data rate, the wireless LAN systems have no provisions for the mobility of an SS or a handoff caused by fast movement of the SS (i.e., cell selection). A communication system based on IEEE (Institute of Electrical and Electronics Engineers) 802.16a communicates through ranging between an SS and a base station (BS). This system will be described with reference to FIG. 1.
FIG. 1 schematically illustrates a conventional OFDM/OFDM-based BWA communication system, particularly an IEEE 802.16a/IEEE 802.16e communication system. However, before describing FIG. 1, it is important to address how the IEEE 802.16a/IEEE 802.16e communication system was developed. The wireless MAN system is a kind of BWA communication system. As compared to the wireless LAN system, it offers wider coverage and supports higher data rates. OFDM and OFDMA (Orthogonal Frequency Division Multiplexing Access) were introduced to the physical channel of the wireless MAN system to support a broadband transmission network. This system is the IEEE 802.16a communication system. The application of OFDM/OFDMA to the wireless MAN system enables the IEEE 802.16a communication system to transmit a physical channel signal over a plurality of sub-carriers, for high-speed data transmission. The IEEE 802.16e communication system, which is an extension of IEEE. 802.16a, introduces nomadic capabilities (mobility of SS). However, the IEEE 802.16e communication system is yet to be specified.
Referring to FIG. 1, the IEEE 802.16a/IEEE 802.16e communication system is configured in a single cell architecture. It comprises a BS 100 and a plurality of SSs 110, 120, and 130 covered by the BS 100. Signal transmission/reception between the BS and the SSs 110, 120, and 130 is performed in OFDM/OFDMA.
FIG. 2 schematically illustrates a downlink frame in the conventional OFDM/OFDM-based BWA communication system, particularly the downlink frame of the IEEE 802.16a/IEEE 802.16e communication system. Referring to FIG. 2, the downlink frame includes a Preamble 200, a Broadcast Control 210, and a plurality of TDM (Time Division Multiplex) fields 220 and 230. The Preamble 200 delivers a synchronization signal, namely a preamble sequence for synchronizing a BS and an SS. The Broadcast Control 210 contains a DL (Downlink)_MAP 211 and a UL (Uplink)_MAP 213. The DL_MAP 211 delivers a DL_MAP message, which includes the information elements (IEs) as shown below in Table 1.
TABLE 1SyntaxSizeNotesDL-MAP_Message_Format( ) { Management Message Type = 2 8 bits PHY Synchronization FieldVariableSee appropriate PHY specification. DCD Count 8 bits Base Station ID48 bits Number of DL-MAP Elements n16 bits Begin PHY Specific Section {See applicable PHY section.  for(i=1; i<=n; i++) {For each DL-MAP element 1 to n.   DL_MAP_Information_Element( )VariableSee corresponding PHY specification.   if!(byte boundary) {    Padding Nibble 4 bitsPadding to reach byte boundary.   }  } }}
In Table 1, Management Message Type indicates the type of the transmitted message, PHY (PHYsical) Synchronization is set according to the modulation/demodulation of the physical channel, for synchronization acquisition, DCD (Downlink Channel Descript) Count is the number of changes in the configuration of a DCD message containing a downlink burst profile, Base Station ID (Identifier) identifies a BS, and Number of DL_MAP Elements n indicates the number of elements following Base Station ID. Although not shown in Table 1, the DL_MAP message further includes information about ranging codes assigned to respective ranging types as will described later in more detail.
The UL_MAP 213 delivers a UL_MAP message includes IEs as shown below in Table 2.
TABLE 2SyntaxSizeUL_MAP_Message_Format( ) { Management Message Type=3 8 bits Uplink channel ID 8 bits UCD Count 8 bits Number of UL_MAP Elements n16 bits Allocation Start Time32 bits Begin PHY Specific Section {  for(i=1; i<n; i+n)   UL_MAP_Information_Element {Variable     Connection ID     UIUC      Offset    }  } }}
In Table 2, Management Message Type indicates the type of the transmitted message, Uplink Channel ID identifies the uplink channel used, UCD (Uplink Channel Descript) Count is the number of changes in the configuration of a UCD message containing an uplink burst profile, and Number of UL_MAP Elements n indicates the number of elements following UCD Count. The Uplink Channel ID is assigned only in a MAC (Media Access Control) sub-layer.
An UIUC (Uplink Interval Usage Code) indicates the usage of an offset set in Offset. For example, if the UIUC is 2, this indicates that a starting offset for initial ranging is set in the Offset. If the UIUC is 3, this indicates that a starting offset for bandwidth request ranging (BW-request ranging) or maintenance ranging (periodic ranging) is set in the Offset. As described above, the Offset indicates the starting offset for initial ranging, BW-request ranging, or periodic ranging according to the information in the UIUC. Information about the feature of a physical channel that delivers the UIUC is provided in the UCD.
If an SS has failed in ranging, it determines a backoff value to increase success probability at a next attempt and retries the ranging after a time delay corresponding to the backoff value. Information needed to determine the backoff value is also provided by the UCD message. The structure of the UCD message will be detailed with reference to Table 3 below.
TABLE 3SyntaxSizeNotesUCD-Message_Format( ) { Management Message Type = 08 bits Uplink channel ID8 bits Configuration Change Count8 bits Mini-slot size8 bits Ranging Backoff Start8 bits Ranging Backoff End8 bits Request Backoff Start8 bits Request Backoff End8 bits TLV Encoded Information for the overall channelVariable Begin PHY Specific Section {  for(i=1; i<n; i+n)   Uplink_Burst_DescriptorVariable  } }}
In Table 3, a Management Message Type indicates the type of the transmitted message, an Uplink Channel ID identifies the uplink channel used, a Configuration Change Count is the number of configuration changes counted by a BS, a Mini-slot Size indicates the size of an uplink physical channel mini-slot, a Ranging Backoff Start indicates the starting point of an initial ranging backoff, that is, the size of an initial backoff window for initial ranging, a Ranging Backoff End indicates the end point of the initial ranging backoff, that is, the size of a final backoff window for the initial ranging, a Request Backoff Start indicates the starting point of a backoff for contention data and requests, that is, the size of an initial backoff window for contention data and requests, and a Request Backoff End indicates the end point of the backoff for contention data and requests, that is, the size of a final backoff window for contention data and requests. The backoff is defined as a time delay value by which the SS waits for a ranging retry, if it fails in any rangings, which will be described in more detail herein below The BS transmits the backoff to the SS. For example, if the Ranging Backoff Start and Ranging Backoff End indicate “10”, the SS attempts the next ranging after a time delay equivalent to 210 (1024) ranging tries.
The TDM fields 220 and 230 correspond to time slots assigned to SSs in TDMA (Time Division Multiple Access). The BS broadcasts necessary information to SSs within its coverage area in the DL_MAP 211 of the downlink frame. Upon a power-on, each of the SSs monitors all frequency bands set for it and detects the strongest pilot channel signal, that is, a pilot channel signal having a highest CINR (Carrier to Interference Noise Ratio). A BS that transmits the highest CINR-pilot channel signal is designated by the SS as its serving BS. Additionally, the SS attains control information about its uplink and the downlink and information about the positions of actual transmit/receive data by checking the DL_MAP 211 and the UL_MAP 231.
FIG. 3 schematically illustrates an uplink frame in the conventional OFDM/OFDMA-based BWA communication system, particularly the IEEE 802.16a/IEEE 802.16e communication system. However, before describing FIG. 3, ranging types, that is, initial ranging, periodic ranging, and BW-request ranging, as provided by the IEEE 802.16a/IEEE 802.16e communication system will be described.
A. Initial Ranging
Initial ranging is performed upon a request from the BS to synchronize with the SS. The purposes of the initial ranging are accurate acquisition of a time offset and adjustment of transmit power between the SS and the BS. Upon a power-on, the SS synchronizes with the BS by receiving a DL_MAP message and a UL_MAP/UCD message, and performs initial ranging to adjust the time offset and transmit power with the BS. Because the OFDM/OFDMA in the IEEE 802.16a/IEEE 802.16e communication system is used, the ranging procedure requires ranging sub-channels and ranging codes. The BS assigns available ranging codes according to the usage, that is, type of ranging.
The ranging codes are generated by segmenting a PN (Pseudorandom Noise) sequence of, for example, (215−1) bits in length in predetermined units. In general, two 53-bit ranging sub-channels form one ranging channel and the ranging codes are produced by segmenting the PN code by the 106-bit ranging channel. Up to 48 ranging codes (RC#1 to RC#48) as constructed in this manner can be assigned to SSs. For each SS, at least two ranging codes as a default are applied to the three types of rangings, i.e., initial ranging, periodic ranging, and BW-request ranging. Thus, different ranging codes are assigned to the three ranging types. For example, N ranging codes are assigned for initial ranging, M ranging codes for periodic ranging, and L ranging codes for BW-request ranging. The SSs are notified of ranging codes assigned to them by the DL_MAP message and perform ranging procedures using the ranging codes in compliance of their usages.
B. Periodic Ranging
After adjusting the time offset and transmit power with the BS by the initial ranging, the SS performs periodic ranging to adjust a channel state with the BS. For the periodic ranging, the SS uses ranging codes assigned for the periodic ranging.
C. BW-Request Ranging
After adjusting the time offset and transmit power with the BS by the initial ranging, the SS requests a bandwidth by ranging, for actual communication with the BS.
Referring to FIG. 3, an uplink frame comprises Initial Maintenance Opportunities 300 for initial ranging and periodic ranging, Request Contention Opportunities 310 for BW-request ranging, and SS Scheduled Data fields 320 including uplink data. The Initial Maintenance Opportunities 300 includes a plurality of access bursts for initial ranging and periodic ranging, and a collision period produced when a collision occurs between the access bursts. The Request Contention Opportunities 310 includes a plurality of bandwidth requests for bandwidth request ranging, and a collision period produced when a collision occurs between the bandwidth requests. The SS Scheduled Data fields 320 include SS1 Scheduled Data to SS N Scheduled Data. An SS transition gap exists between adjacent SS Scheduled Data fields.
FIG. 4 is a flowchart illustrating a ranging procedure between an SS and a BS in the conventional OFDM-based BWA communication system. Referring to FIG. 4, upon a power-on, an SS 400 monitors all predetermined frequency bands and detects a pilot channel signal with a highest CINR. Considering a BS 420 that transmits the detected pilot channel signal as a serving BS, the SS 400 acquires system synchronization with the BS 420 by receiving the preamble of a downlink frame from the BS 420.
As described above, with the system synchronization acquired, the BS 420 transmits DL_MAP and UL_MAP messages to the SS 400 in steps 411 and 413. As illustrated in Table 1, the DL_MAP message provides information needed for the SS 400 to synchronize with the BS 420, and information about the configuration of a physical channel that delivers messages to the SS 400 on the downlink after synchronizing. As illustrated in Table 2, the UL_MAP message provides the SS 400 with information about a scheduling period for the SS 400 and the configuration of an uplink channel.
The BS 420 periodically broadcasts the DL_MAP message to all SSs. When the SS 400 is capable of receiving the DL_MAP message continuously, it is said that the SS 400 is synchronized to the BS 420. That is, SSs, which have received the DL_MAP message, can receive all downlink messages.
As illustrated in Table 3, the BS 420 transmits to the SS 400 a UCD message containing information about a backoff value in case of access failure.
For ranging, the SS 400 transmits a Ranging. Request (RNG_REQ) message to the BS 420 in step 415. In step 417, the BS 420 then transmits to the SS 400 a Ranging Response (RNG_RSP) message containing information required to adjust frequency, time offset, and transmit power.
The RNG_REQ message is formatted as illustrated in Table 4 below.
TABLE 4SyntaxSizeNotesRNG-REQ_Message_Format( ) { Management Message Type = 48 bits Downlink Channel ID8 bits Pending Until Complete8 bits TLV Encoded InformationVariableTLV specific}
In Table 4, a Downlink Channel ID indicates the ID of the downlink channel that has delivered the UCD message to the SS 400. Pending Until Complete indicates the priority level of an RNG_RSP message for the RNG_REQ message. If the Pending Until Complete is 0, a previous RNG_RSP has higher priority. If the Pending Until Complete is not 0, an RNG_RSP for the current RNG_REQ has priority over an RNG_RSP for any other RNG_REQ.
The RNG_RSP message in response for the RNG_REQ message illustrated in Table 4 is formatted as illustrated in Table 5 below.
TABLE 5SyntaxSizeNotesRNG-RSP_Message_Format( ) { Management Message Type = 58 bits Uplink Channel ID8 bits TLV Encoded InformationVariableTLV specific}
In Table 5, an Uplink Channel ID is the ID of the uplink channel that has delivered the RNG_REQ message.
Herein below, a ranging procedure in an OFDMA-based IEEE 802.16e communication system will be described below. In the OFDMA-based IEEE 802.16e communication system, instead of transmitting the RNG_REQ message, a dedicated region for ranging is set and a ranging code is transmitted in the dedicated region, to thereby perform the ranging more efficiently.
FIG. 5 is a flowchart illustrating a ranging procedure between an SS and a BS in the conventional OFDMA-based BWA communication system. Referring to FIG. 5, upon a power-on, a BS 520 transmits DL_MAP and UL_MAP messages to an SS 500 in steps 511 and 513, respectively. Steps 511 and 513 are performed in the same manner as steps 411 and 413 of FIG. 4. Thus, they are not detailed here. Instead of transmitting the RNG_REQ message as illustrated in FIG. 4, the SS 500 transmits a ranging code to the BS 520 in step 515. The BS 520 transmits an RNG_RSP message for the ranging code to the SS 500 in step 517.
In relation to the ranging code, the RNG_RSP message further includes the following:
1. Ranging Code: a received ranging CDMA code;
2. Ranging Symbol: the OFDM symbol in which the ranging CDMA code was received;
3. Ranging Sub-channel: the ranging sub-channel in which the ranging CDMA code was received; and
4. Ranging Frame Number: the frame number in which the ranging CDMA code was received.
As described above, the IEEE 802.16a communication system works for fixed SSs in a single cell structure with no regard to the mobility of the SSs. Further, the IEEE 802.16e communication system adds SSs' mobility to the IEEE 802.16a communication system. Therefore, it must support the mobility in a multi-cell environment. To do so, the operations of the SS and the BS must be modified. However, no specific proposals have been made for the multi-cell and the SSs' mobility for the IEEE 802.16e communication system. Therefore, there is a pressing need for providing a handoff for an SS under a multi-cell environment in order to support the mobility of the SS in the IEEE 802.16e communication system.